Add more detail (e.g. line number) to various "error opening" log msgs

Some of the errors could be pretty vague since they looked the same and
we were depending on the path/file attempted to be opened in the log msg
to clue us into what exactly was being attempted. But some of these file
paths are sysop-configurable, and they can put unexpected things in there
that make it difficult to determine exactly what file was trying to be
opened and for what purpose. This should help a little bit.